This invention concerns a thread cutter for chainstitch sewing machine, particularly one having a swing arm to move directly a hook rod arm and a cutter arm.
A known conventional thread cutter for a chainstitch sewing machine shown in FIG. 6, disclosed in a U.S. Pat. No. 4,098,209, includes a driving unit (not shown in the figure), a swing arm 10 moved by the driving unit to swing clockwise for a proper angle and then swing back to perform a round of thread cutting action. At the beginning of the action, a front head 101 of the swing arm 10 fits in notch 201 of a hook rod arm 20. As the swing arm 10 is moved to swing, the front head 101 moves down in the notch 210 so that the hook rod arm 20 is pushed down, guided by a screw 203 in a slot 202, moving down slantingly. During downward movement of the hook rod arm 20, it contacts a high point 301 of the cutter arm 3, forcing the cutter arm 30 also move along with the hook rod arm 20 slantingly. During continuous forward movement of the swing arm 10, the hook rod arm moves forward straight, guided by the slot 202 sliding along the screw 203, and then a hook rod 40 fixed with the hook rod arm 20 moves to let its front hook 401 and its rear hook 402 hook related threads, the hook rod arm then further moves forward, but the cutter arm 30 is to be stopped by means of the screw 303 blocking an upper end of the slot 302. So the cutter 50 fixed on the front end of the cutter arm 30 does not advance.
Next, the swing arm 10 is moved to swing back, retreating the hook rod arm 20, letting the hook rod 40 pass by the cutter 50 to cut the threads. Then the hook rod arm 20 is moved back to its original position, and the cutter arm 30 is moved back by a spring 304 lengthened, with the high point 301 not pushed by the hook rod arm 20. The thread cutter just described has the swing arm 10 directly moving the hook rod arm 20 forward and backward, and the cutter arm 30 is moved indirectly by means of the high point 301 being pushed by the hook rod arm forward and being retreated by the spring 304. Therefore, the cutter arm is indirectly moved back and forth by the swing arm, which gives rise to complicated structure of the conventional cutter, in addition to disorder, and wear and tear of connecting spots such as the high point 301 and the spring 304.